Help me

I have bought used iphone 6s 64g. When I checked the serial number on the website https://checkcoverage.apple.com/en/en/, it was reported by Apple: "We're sorry, but this is a serial number for a product that has been replaced. Your information and re-enter your serial number. If your information is correct, you may need to contact us. " I am very puzzled not knowing that my iphone phone is anything but. Can you help me know what happened to my iPhone? Thank you!